Thermal switch with expanding cycling control



23, 1955 E. G. FRANKLIN 2,716,172

THERMAL SWITCH WITH EXPANDING CYCLING CONTROL Filed Sept. 12, 1952 2Sheets-Sheet 1 :28 i I36 I05 94 I 45 INVENTOR EDMOND G. FRANKLIN BATTORNEY Aug. 23, 1955 E. G. FRANKLIN 2,716,172

THERMAL SWITCH WITH EXPANDING CYCLING CONTROL Filed Sept. 12, 1952 2Sheets-Sheet 2 INVENTOR EDMOND G. FRANKLIN BY W ATTORNEY THERMAL SWITCHWITH EXPANDING CYCLING CONTROL Edmond G. Franklin, Minneapolis, Minnassignor to General Mills, Inc, a corporation of Deiaware ApplicationSeptember 12, 1952, Serial No. 309,325

19 Claims. (Cl. 200-137) The present invention relates to thermalswitches and more particularly to an improved mechanism for delaying thecycling of such a switch and improving the engagement and disengagementof the switch contacts.

In many thermal switches of the prior art, the response of the thermalportion of the switch to the temperatures being controlled is so rapidthat the switch contacts barely have an opportunity to complete thecircuit before they are again disengaged. As a result, such switchestend to cycle very rapidly and may therefore be unsatisfactory for usein applications where slower cycling is desirable. For example, in thehome appliance field there are certain standards, such as those of theNational Electrical Manufacturers Association, concerning the maximumnumber of permitted cycles per unit of time. Furthermore, such rapidcycling and indecisive engagement and disengagement of the contacts mayresult in chattering of the contacts at the make or the break and thuscause arcing with resultant wear and erosion of the contact surfaces.

Proposals have been made previously for the solution of such problems,but such prior proposals have generally involved the mounting of one ofthe switch contacts on an auxiliary strip of bimetal to be heated anddeflected by the passage of current through the contact. These deviceshave the difiiculty, however, that if the bimetal is made sufiicientlysmall in cross section to obtain rapid heating and deflection, theauxiliary bimetal will be so thin that it will not offer firm oradequate support for the contact in many cases. Such relatively smallcross sections of bimetal will also yield only small deflection forces,so that particularly on engagement of the contacts there will be littleincrease in the contact pressure to prevent chattering.

With these problems and constructions of the prior art in view, it isone object of the present invention to provide an improved cycling delaymechanism for a thermal switch.

Another object is the provision of a cycling delay mechanism which willact rapidly and effectively to increase the contact pressure when thecontacts first engage and to increase the gap when the contacts firstdisengage. A further object is the provision of such a cycling delaymechanism in which no auxiliary bimetallic member is required.

Still another object is the provision of a cycling delay mechanism inwhich use is made of the rapid expansion and contraction of arestraining strip or wire of relatively small cross section.

A still further object is the provision of an improved switchconstruction in which one of the contact arms includes a contactsupporting portion and a base portion, the contact support beingresiliently biased to a given position with respect to the base portionand then being held out of said normally biased position by means of arestraining member which, when cold, holds the contact farther away fromits opposing contact than its normally biased position.

2 ,716,172 Patented Aug. 23, 1955 Other objects and advantages will beapparent from the following specification in which certain embodimentsof the invention have been described with particular reference to theaccompanying drawings. In these drawmgs,

Figure 1 is a side elevation, partly in section, showing a preferredembodiment of the present invention, as applied to a fiatiron or otherheating plate. In this figure, the parts are shown at the instant whenthe contacts first engage each other during cycling.

Fig. 2 is a schematic view, similar to Figure 1, showing the position ofthe parts immediately after the contacts have engaged.

Fig. 3 is a view similar to Figs. 1 and 2, but with the parts in theposition wherein the contacts have just barely disengaged each other.

Fig. 4 is a schematic view similar to Fig. 3 showing the action of theswitch contact arm according to the invention immediately after theinstant of disengagement of the contacts.

Fig. 5 is an exploded partial perspective view in brackets showingdetails of construction of one of the contact arms and its restrainingmember.

Fig. 6 is a view similar to Fig. 1 showing another embodiment of theinvention.

Fig. 7 is a partial perspective view in brackets similar to Fig. 5showing the construction of the contact arm and restraining member ofthe device of Fig. 6.

Fig. 8 is a chart showing operation of a device similar to that of Fig.1 but without the cycling delay mechanism of the present invention, and

Fig. 9 is a chart showing the cycling of the specific preferredembodiment of Figs. 1 to 5.

With reference to the construction shown in Figs. 1 through 5, whichconstitute a preferred embodiment of the present invention, the thermalswitch is designated generally at 20. The switch includes a supportingbracket 22 having a horizontal top wall 24, spaced vertical side walls26, and horizontal bottom flanges 23. These bottom flanges may besecured by screws or bolts 30 to appropriate portions 32 of the deviceto be controlled. Here this device is illustrated in the form of afiatiron soleplate 34 having a cast-in heating element 36 connected incircuit to be controlled by the switch 20 in known manner.

In the switch itself, a vertical supporting sleeve 38 depends from thetop wall 24 of the supporting bracket and is secured thereto by havingits upper end riveted over onto the wall as indicated at 40. Extendinghorizontally within bracket 24 and supported by the post or sleeve 38are the upper and lower contact arm assemblies 42 and 44, respectively.These contact arms are insulated from the post 38 and supporting bracket22 by suitable insulating members and inserts such as shown at 46 and48. Part of the base portion of each contact arm extends rearwardly toprovide terminal connections as indicated at 50 and 52 respectively.

The lower contact arm assembly 44 includes a base or main body portion54 and a longitudinally overlapping extension 56 which projects beyondthe portion 54. A

contact 58 is carried at the outer end of extension 5'6.

To secure the two contact arm plate portions 54 and 56 together, theplate 56 is provided at its inner end with an upturned lug or projection60 which projects through an opening 62 in the base portion 54 of thearm to prevent relative movement at this point. The two portions 54 and56 are further connected to each other by an insulating member 64 andstud 66 which pass therethrough and provide a means for attachment ofthe thermally responsive actuating portion of the switch. Thus the twoswitch blade portions 54 and 56 are interconnected at two longitudinallyspaced points to prevent relative slippage between these portions andpermit them to operate in effect as a single contact arm.

While various types of thermally responsive means can be used to engagethe insulator 64 and stud 66 to actuate the lower contact arm, theinvention has been illustrated in connection with means for connectingstud 66 to a portion of the fiatiron remote from the point of support ofthe switch. Thus, the lower end of stud .66 passes through an opening 68in a connection plate 70 held in position against the bottom ofinsulator 64 by a riveted end 72 of stud 66. Connection plate 70 carriesa pin or stud 74 pivoted to one end 76 of a shaft or rod 78. The otherend 80 of this shaft 78 is secured by bolt 82 to a remote portion 84 ofthe flatiron or other heated device. If a connecting member 78 is chosenhaving a lower coefficient of thermal expansion than that of the heatingdevice 34, it will be apparent that the lower contact arm assembly 44will be deflected downwardly in response to heating and expansion of thedevice 34 by heating element as.

Cooperating with the lower contact 58 is an upper contact 86 having anupwardly projecting stud or shaft 88 secured thereto and connecting thecontact 86 to the upper contact arm assembly 42. This upper contact armassembly is shown in part in Fig. 5 and includes primarily a contact arm90 and a restraining and conducting member 92. In this embodiment of theinvention, contact arm 90 has a base portion 94 provided with an opening96 through which the insulator 46 and supporting post 38 of Fig. Iextend to support the arm. The base portion of the contact arm alsoincludes an intermediate portion 98 projecting outwardly from thesupporting post, and an outer contact supporting portion 190.

The respective portions are resiliently biased to predetermined initialpositions with respect to each other. For this purpose the contact armis conveniently made of resilient or spring material capable of taking acertain set or stress. In the example illustrated in Figs. 1 to 5, theintermediate portion 98 is initially biased to a deflected position withrespect to the base portion 94, this displacement or bias being in adirection tending to move the outer contact supporting portion 100 awayfrom the lower contact arm. At the same time, the outer contact support100 is initially deformed at 104 to a pre-biased or normal positiondisplaced from the plane of intermediate portion 98 in a directiontoward the remaining contact arm of the switch.

According to a further feature of the invention, the intermediatesection 93 of the contact arm 90 is provided with strengthening means,such as the parallel upwardly extending flanges 106, to maintain thisintermediate portion in a substantially rigid and straight conditionduring operation of the switch. The base portion of the contact armincludes an opening 108 for attachment of one end of the restrainingmember at a point spaced substantially inwardly from the contact supportltlti. Here this opening 108 is in the intermediate portion of the baseportion. The contact support also includes an opening 110 through whichthe vertical stud 88 of contact 86 may extend to support such contact inassembled relation.

As shown also in Fig. 5, the restraining member 92 of the presentinvention includes an inner end 112 having an opening 1114 designed foralignment with the opening 108 in the contact arm 94). The outer end 116of the restraining member 92 also includes an opening 118 adapted foralignment with the opening 110 of the contact support 100. Theintermediate portion of restraining member 92 is cut away as shown at119 through a substantial length and width of the device to providespaced strips 120 at each side of member 92 extending over a substantiallongitudinal distance between the openings 114 and 118. The size ofopening 119 must be great enough so that the total cross section of theremaining metal in the side strips 120 is relatively small withreference to the current to be passed through the contacts 86 and 58,and with reference to the total cross section of. the

contact arm 90. Thus, as discussed below, the passage of current throughthe restraining member 92 will result in substantial heating andexpansion of the side strips 119 and 120 to achieve the desiredobjectives of the present invention.

The upright stud 88 on upper contact 36 passes through the opening 110in the contact support 100 and through the opening 118 in the outer end116 of restraining strip 92 and is riveted over at 122 to hold the partsin assembled relation. An intermediate insulating member and spacer 124also extends around the stud 88 at the point of passage through opening110 to cooperate with insulating washer 126 and support the contact 86in completely insulated relation with respect to the supporting portion100 and the other portions of contact arm 90. The upper end of stud 88,however, is in direct engagement with the portion 116 of restrainingmember 92 and is thus electrically connected thereto. At the other endof the restraining member 92, the rivet 128 passes through opening 114in the restraining member and through opening 108 in the contact arm andprovides both a mechanical and electrical connection between the partsat this point.

The length of restraining member 92 between openings 114 and 118 is sochosen that when the parts are assembled as in Fig. 1 and therestraining member is in its cold or unexpanded position, i. e., when nocurrent passes through it, the restraining member will be connectedunder longitudinal tension between the contact support portion and baseportion and will hold the upper end of stud 83 and the contactsupporting portion 100 in a position which is upwardly displaced fromthe normal resiliently biased position of the portion 100. In thisinstance, the eifect of the restraining member is to deflect the contactsupporting portion 100 upwardly away from the lower contact 58 until theportion 100 lies in substantially the same plane as the intermediateportion 98 of the contact arm. Thus the contact portion 100 is in efiecthinged at 104 by virtue of the cut-away portion 165 and is normallyresiliently biased to its first position as shown in Fig. 5, but held bythe restraining member 92 in its second position as shown in Fig. 1.This second position is displaced from the first position in a directionaway from the other or second contact.

To adjust the operating temperature maintained by switch 20, anadjusting shaft 130 is provided. This shaft has its lower end threadedat 132, and the threaded end engages within a threaded boss 134 in thetop wall 24 of the supporting bracket 22. An insulating button 136within control shaft 130 engages the rigid intermediate portion 98 ofthe upper switch contact arm assembly 42, and extends between therestraining member strips 120.

The position of this insulating button 1.36 within the control shaft 130may be calibrated by an internal calibration screw 138. A collar 140 isthreaded on the end of the control shaft 130 and carries a projectingstep 142 for engagement with a lug 144 struck up from the wall 24 tolimit the operating range of the device. Thus rotation of the controlshaft 130 will cause relative vertical movement of the insulating button136 to determine the normal operating position of the intermediateportion 93 of the upper contact arm. This portion of the contact arm isresiliently biased upwardly by virtue of the initial resilient stress orbend at 162 (Fig. 5) so that the contact arm will at all times beresiliently urged upwardly against the insulating button 136 to assumewhatever adjusted position is determined by the position of controlshaft 130. This adjustment is independent of any relative movement ofcontact support portion 100 between its first and second positions, inthis preferred form of the invention.

With the above construction of the parts in mind, the p r t n of thedevice may now be described. Figure is the position just at the end ofthe cooling portion ofthe cycle before any substantial current has hadan opportunity to flow through the contacts 86 and 58. The contactsupport portion is thus in its second position with reference to thebase portion of the contact arm. Fig. 2 illustrates in heavy lines thecondition of the parts at an instant after the position of Fig. l, i.e., after the initial passage of current has caused substantial heatingan dexpansion of the restraining strip portions 120. This expansion haspermitted relative downward rocking of the contact supporting portion100 and its associated contact 86 from the dotted line position of Fig.2 (which is the same as the second position, shown in Fig. l) to theheavy line position of Fig. 2 in response to the normal resilient biasof the contact portion 190 at the line of deflection 104. Thus the heavyline position of Fig. 2 shows the contact support portion in its firstposition, which is relatively displaced from the second position in adirection toward the second contact and will therefore increase thecontact pressure and delay the subsequent disengagement of the contacts.

Passage of current through the switch and through the associated heatingelement 36 will then heat the flatiron or other device 34 and cause thelatter to expand with respect to the member 78. When the relativedifference in expansion between the device 34 and member 78 reaches acertain point, the resulting deflection of the stud 66 and downwardmovement of associated lower contact portions 54 and 56 will pull thelower contact 58 downwardly until the contacts just begin to disengageas illustrated in Fig. 3. Until this instant of disengagement there hasbeen no interruption in the current through the restraining strip 92 andthus the upper contact arm assembly has remained in the heavy line orfirst position of Fig. 2.

Immediately after the contacts separate as shown in Fig. 3, however, theinterruption of current will termimate the heating eitect 011 therestraining member 92 and will cause its longitudinally extending strips120 to contract rapidly. This contraction will tend to restore the uppercontact arm assembly from the dotted line position of Fig. 4 to theheavy line position of the figure, in which the upper contact arm issubstantially in the second position of Fig. 1. Because of this upwardmovement of the upper contact 86, it is clear that the gap between thecontacts will be increased and that the lower contact will then have tomove upwardly a greater distance before the contacts can re'engage. Thuseven though the current to heating element '36 is interruptedimmediately, and even though the contact 58 starts reasonably promptlyto move upwardly as the soleplate cools and contracts with reference tothe connecting bar '78, there will be a substantial time interval beforethis cooling action is sufli'cient to permit reengagement of thecontacts in the original position of Fig. 1.

In other words, the construction shown in Figs. 1 to 5 provides a meansfor increasing the contact pressure and thus delaying disengagement ofthe contacts (as shown in Fig. 2) as a result of current passage oninitial engagement of the contacts. The construction also provides ameans for increasing the contact gap and thus increasing the timerequired for reengagement of the contacts after the current isinterrupted by initial disengagement of such contacts. As a result, theaction of the switch on engagement and disengagement of the contacts ismuch sharper and cleaner than would otherwise be the case, andunnecessary wear and erosion of the contacts is eliminated. Furthermore,the period of cycling is lengthened to the desired extent and the tem- 6perature of the device 34 will, in many cases, be much more uniformlycontrolled.

Figs. 6 and 7 illustrate another embodiment of the invention in whichthe upper contact arm assembly includes two separate contact arm partsinstead of a single integral contact arm strip as in Figs. 1 to 5.

The switch support 146 is similar to that of the first embodiment and issecured at 148 to one portion of the flatiron soleplate 150 or otherdevice to be controlled. The heating element 152 is similarly connectedto heat the plate 150. A depending stud 154 on the supporting bracket146 carries upper and lower or first and second contact arm assemblies156 and 158, respectively. These contact arm assemblies in turn carryfirst and second upper and lower contacts 160 and 162. The lower contactarm assembly 158 is similar to that previously described and includes anouter leaf portion 164 and an inner base portion 166 which areinterconnected with each other by means of a stud 168 which alsosupports a connection plate 170. Stud 172 connects the connection plateto one end of the bar 174-, While the other end of the latter isconnected at 178 to a remote portion of the device 150 as previouslydescribed. Diiterential expansion between the soleplate 150 and member174 will accordingly pull the contact 162 downwardly when the soleplateis heated and permit contact 162 to move upwardly when the soleplate iscooled.

The main portions of the upper contact arm assembly 156 are shown inFig. 7. Here the assembly includes an inner contact arm base portion 18)provided with an opening 182 for assembly with the stud 154 andassociated insulating members. This base portion includes an outwardlyprojecting intermediate portion 184 which is resiliently pre-biasedupwardly from the plane of the base portion along a line of deformationat 186. At the outer end of this intermdiate portion 184 is a resilienthinge means for the outer contact support portion 190. This hinge meansincludes a depending flange 138 having an opening therein for connectionto the contact support portion. The outer contact arm portion 1% has asimilar depending flange 192 at its inner end provided with an opening194 through which a rivet 196 passes to connect the flanges 188 and 192.Insulating members 198 completely surround the rivet 196 and separatethe flanges 188 and 192, so that the outer contact support portion 190,which supports the upper contact 16%, is completely insulatedelectrically from the base portion of the contact arm including theintermediate portion 184. Since the contact 160 is supported by outercontact support portion in electrically insulated relation with respectto the inner contact arm parts, it is unnecessary to insulate thecontact 160 specifically from the contact supporting arm portion 190,the necessary insulation in this respect being provided at 198.

The restraining member according to this embodiment of the invention isshown at 202 and involves a single strip of relatively small crosssection, one end of the strip having an opening 204 for engagement witha stud on the upper contact 160 to connect the restraining member 202electrically and mechanically with the upper contact 160. The inner endof this restraining member 202 has an opening 207 adapted to receive arivet 208 which will also pass through an opening 210 in theintermediate portion 184 of the contact arm. Thus the restraining strip202 will provide an electrical connection between the contact arm baseportions which are connected in circuit with the heating element 152,and the outer contact support portion 196 with its contact 160.

According to the present invention, the dimensions of the parts are sochosen that the restraining strip 202, when cold, will be underlongitudinal tension and will hold the outer contact supporting portion190 of the contact arm assembly in a second position which is displacedupwardly, i. e., away from the second or lower contact 162, with respectto the normally biased first position of the portion 190 as shown inFig. 7. For example, one or both of the depending flanges 188 and 192 isinitially formed with such an angle that the outer contact supportingportion 190 will extend downwardly below the plane of the intermediatecontact arm portion 184, in the absence of restraining member 202. Asshown in Fig. 6, the restraining member 202 eifectively holds the outercontact supporting portion 190 up in a more horizontal plane so that itappears in Fig. 6 substantially as an extension of the plane of theintermediate portion 184. An insulating member 212 is located betweenthe restraining member 202 and the remaining portions of the contact armassembly to insure that the current to the contact 160 will all passthrough the restraining and conducting member 202.

For adjustment of the device of Fig. 6, a control shaft 214 is providedjust as in the previous case. This control shaft is threaded into a boss216 on the support 146 and carries an insulating member 218 whichengages the upwardly biased contact arm and holds it down to the desiredposition. In Fig. 6, the position of the insulating button 218 is suchthat the intermediate contact arm portion 134- appears substantially asa coplanar extension of the part of base portion 180 which is engaged bythe supporting insulators, thus eflectively straightening out the bendwhich normally appears at 186. This pre-bias of the portion 134 thusserves to hold the upper contact arm assembly against the insulatingbutton 218 at all times to establish the desired operating temperaturefor the switch.

Operation of the device of Figs. 6 and 7 will be essentially the same asthat illustrated in Figs. 1 to 5, except that the relative displacementof the contact 160 upwardly and downwardly will be a swinging movementfrom the region of the flanges 188 and 192 and adjusting stop 218 ratherthan solely a bending of the contact supporting portion 190 itself.

While both the embodiments of Figs. 1 to 5 and of Figs. 6 and 7 haveprovided the expanding means for cycling delay in connection with thefirst or upper contact arm, it will be apparent that the combination ofprebiased contact arm and restraining and conducting member of smallcross section could, in many cases, be associated with the lower orsecond contact arm, i. e., the same contact arm which is actuated by thethermally responsive portion of the switch assembly provided thethermally responsive connection to the arm is at some point other thanthe relatively movable contact support portion.

As pointed out above, the actual total cross section of the restrainingmember must, in any case, be small enough to insure substantial heatingby the particular current passing through it. Also, the length of therestraining member must be great enough to provide substantial totallongitudinal expansion of the restraining member and consequent lateraldisplacement of the contact in response to such heating. In general,some of the benefits of the present invention, such as increased contactpressure and cleaner make and break of the circuit, can be achieved evenwhere such relative displacements are small. Preferably, however. therelative contact displacement should be of the order of at least two tofour mils when used on 60 cycle A. C. at 115 volts with currents of theorder of 9 to 10 amperes. Higher A. C. voltages would require stillgreater relative displacement to insure positive elimination of arcing.

While the specific dimensions will thus depend on the current-carryingcharacteristics of the material, the current to be controlled, and thethermal expansion characteristics of the restraining member, thefollowing example illustrates one set of operating conditions and switchdimensions which will achieve the desired result.

Example 1 In a switch constructed in the form shown in Figs. 1 through5, the current to be passed through the switch contacts and restrainingmember was of the order of 10 amps. In this case the restraining memberwas made of a nickel chromium alloy commercially available as Nichrome#5, and comprising approximately Ni and 20% Cr. The effective length ofthe strips of limited cross section was 0.876 inch, the thickness ofeach strip being 0.012 inch and the width of each strip 0.042 inch. Thisrestraining member was connected to the contact stud above the contactsupport portion and spaced from such portion by an insulator whose axialthickness was 0.25 inch. The distance along the contact arm from thevertical center line of this contact stud to the hinge line between thecontact support portion and intermediate portion was also 0.25 inch.

With these dimensions and currents, the relative change in verticaldisplacement of the upper contact resulting from interruption orreestablishment of current was sufficient to achieve the objectives ofslower and more even cycling according to the invention.

According to the foregoing description, a thermal switch has beendescribed which substantially accomplishes the objects set forth at thebeginning of this specification. The desired cycling delay and clean-cutengagement and disengagement of the contacts are secured by a devicewhich requires no auxiliary bimetallic strip. The contacts are at alltimes supported by contact arm members of substantial cross section andstrength, and the relative expansion and contraction of the restrainingand conducting member of small cross section are used merely to controlthe deflection of the outer contact arm portions, without having tosupport the contact itself.

Figs. 8 and 9 illustrate the advantages of operation of the presentdevice. Fig. 8, for example, shows a typical chart of temperatureagainst time during cycling of a flatiron substantially similar to thatof Figs. 1 to 5, but without the cycling delay mechanism of the presentinvention. Here it is apparent that the frequency of engagement anddisengagement of the contacts is relatively high and that the peaks 222of the temperature curve 220, as well as the valleys 224, are far fromuniform in temperature range. In contrast, the temperature curve 226 ofFig. 9 (which is typical of the embodiment of Figs. 1 to 5) shows asubstantially greater time interval between successive cycles and alsoshows relatively uniform temperatures for the peaks 228, on the onehand, and the valleys 230, on the other.

Not only does the construction of the present invention provide thedesired cycling delay to eliminate chattering of the contacts, but theparticular pre-bias and pivotal deflection of the contact supportingportion of the device, particularly in the preferred embodiment of Figs.1 to 5, provides a rocking action at the make and break which isparticularly helpful in securing firm initial engagement of the contactsand clean-cut disengagement thereof at the appropriate portions of thecycle.

Since minor variations and changes in the exact details of constructionwill be apparent to persons skilled in this field, it is intended thatthis invention shall cover all such changes and modifications as fallwithin the spirit and scope of the attached claims.

Now, therefore, I claim:

1. A thermal switch comprising first and second contact arms carryingfirst and second contacts respectively, the first contact arm having abase portion and a contact support portion, means movably mounting thecontact support portion with respect to the base portion for relativemovement between a first position toward the second contact and a secondposition displaced from the first position in a direction away from thesecond contact, the first contact being carried by said contact support,and a longitudinally expanding and contracting restraining memberconnected between said contact support and said base portion andcontrolling the relative movement of the contact support, saidrestraining member having a portion of substantial length and smallcross-section connected in circuit for passage of current through saidportion when the contacts are engaged, said cross section being smallwith respect to the current passed therethrough thereby causingsubstantial heating and longitudinal expansion of the restraining memberin response to engagement of the contacts and substantial cooling andcontraction of the restraining member in response to separation of thecontacts, said expansion of the restraining member acting in a directionto effect movement of the contact support from second to first positionand said contraction of the restraining member acting in a direction toeffect movement of the contact support from first to second position.

2. A thermal switch according to claim 1 in which the first contact issupported in electrically insulated relation to said base portion andsaid restraining member constitutes the sole electrical connectionbetween the first contact and base portion.

3. A thermal switch comprising first and second contact arms carryingfirst and second contacts respectively, the first contact arm having acontact support portion carrying said first contact and supported forlimited movement toward and away from the second contact between a firstposition toward said second contact and a second position displaced fromthe first position in a direction away from the second contact, saidcontact support portion being resiliently biased toward its firstposition, and a restraining member connected under longitudinal tensionbetween the contact support portion and another part of the switch andnormally holding the contact support in said second position againstsaid resilient bias, a portion of said restraining member beingelectrically connected in circuit with one of said contacts and having asubstantial length and relatively small cross section such that passageof current through said restraining member portion on engagement of thecontacts expands said portion substantially and thereby permitsresiliently biased movement of the contact support and first contacttoward said first position, increasing the contact pressure and delayingthe subsequent disengagement of the contacts.

4. A thermal switch according to claim 3 having thermally responsivecontrol means operatively connected to one of said arms and causingengagement and disengagement of the contacts in response topredetermined operating temperatures, and manual control meansoperatively connected to the other contact arm for adjustment of thecontact arm position to determine the operating temperature of theswitch, one of said control means engaging the first contact arm at apoint other than the contact support portion, and said restrainingmember extending generally along the contact arm and having twolongitudinally extending laterally spaced portions of small crosssection, between which portions said one of said control means extends,both of said spaced portions being electrically connected in circuit forheating and expansion as described.

5. A thermal switch comprising first and second contact arms carryingfirst and second contacts respectively, the first contact arm having abase portion by which the arm is supported, a contact support carryingsaid first contact and resiliently hinged to said base portion forlimited movement toward and away from the second contact between aresiliently biased first position toward said second contact and amechanically restrained second position displaced from the firstposition in a direction away from the second contact, and a restrainingmember connected between the contact support and base portion andnormally holding the contact support in said restrained second position,at least one portion of said restraining member being electricallyconnected in circuit with one of said contacts having a substantiallength and relatively small cross section such that passage of currentthrough said restraining member portion on engagement of the contactsexpands said portion substantially and thereby permits resilientlybiased movement of the contact support and first contact toward saidfirst position, increasing the contact pressure and delaying thesubsequent disengagement of the contacts.

6. A thermal switch according to claim 5 having thermally responsivecontrol means operatively connected to one of said arms and causingengagement and disengagement of the contacts in response topredetermined operating temperatures, and manual control meansoperatively connected to the other contact arm for independentadjustment of the contact arm position to determine the operatingtemperature of the switch, one of said control means engaging the baseportion of the first contact arm midway between the lateral edges of thearm, and said restraining member having two longitudinally extending,laterally spaced portions of small cross section, one at each side ofthe control means, both of said spaced portions being electricallyconnected for uniform heating and expansion to move the contact supportportion evenly from second to first position.

7. A thermal switch according to claim 6 in which the thermallyresponsive control means is connected to the second control arm and themanual control means engages the first contact arm between saidportions.

8. A thermal switch comprising first and second contact arms, first andsecond contacts carried respectively by said arms, and thermallyresponsive control means operatively connected to one of said arms andcausing engagement and disengagement of said contacts in response tovariations from a predetermined temperature to be maintained, the firstcontact arm having a base portion, a contactsupporting portion connectedto the base portion and resiliently biased to a given position withrespect to the base portion and supporting the first contact ininsulated relation with respect to the base portion, and a restrainingmember electrically connecting the contact and base portion, saidrestraining member, when cold, normally holding said contact-supportingportion out of said resiliently biased position in a direction away fromthe second contact, and said restraining member having a relativelysmall cross section with reference to the total cross section of the armand the current passing through the contacts, passage of such current onengagement of the contacts thus causing substantial heating andexpansion of the restraining member and permitting movement of thecontact-supporting portion back toward its biased position therebyincreasing the contact pressure and delaying disengagement of thecontacts, and interruption of the current on opening of the contactscausing cooling and contraction of the restraining member and moving thecontact-supporting portion away from its biased position therebyincreasing the gap between the contacts and delaying reengagement of thecontacts.

9. A thermal switch according to claim 8 in which said first contact armincludes a relatively straight and rigid intermediate portion betweenthe contact-supporting portion and base portion, and manual controlmeans operatively engaging one contact arm and establishing the normalposition of said arm to determine the temperature at which the contactsopen and close, one of said control means engaging the rigidintermediate portion of th first contact arm.

10. A thermal switch according to claim 9 in which said intermediateportion is resiliently biased with respect to the base portion in adirection tending to move the intermediate portion and first contact asa unit away from the second contact, said one of said control meansengaging the rigid intermediate portion on the side away from thecontacts and thereby limiting the resiliently biased movement of theintermediate portion to control the operating position of the contact.

11. A contact arm subassembly for a thermal switch, comprising aresilient contact arm having a base portion adapted to engage a support,said base portion including an intermediate portion extending outwardlyfrom the base portion beyond the support, a contact support portion atthe outer end of the intermediate portion, means providing a resilienthinge between the contact support portion and intermediate portion forlimited relative rocking movement of the contact support between firstand second positions displaced from each other in a direction generallynormal to the plane of the arm, said resilient hinge means normallybiasing the contact support portion toward its first position, a contactmounted on that face of the contact support portion which is oppositethe direction of movement of the contact support portion from first tosecond position and a restraining member located beyond the oppositeface of the contact support portion, said restraining member extendinggenerally along the contact arm and connected in longitudinal tensionbetween the contact support portion and base portion, the restrainingmember (when cold) normally holding the contact support portion in itssecond position against the bias of said resilient hinge means, meansconnecting a portion of said restraining member electrically in circuitwith said contact, and said restraining member portion having asubstantial length and a small cross section with respect to the currentto be passed through it, passage of such current thereby causingsubstantially rapid heating and expansion of the member for resilientlybiased movement of the contact portion from second to first position.

12. A contact arm subassembly according to claim 11 in which the baseportion, intermediate portion and contact support portion are formed ofa single integral strip of resilient material.

13. A contact arm subassembly according to claim 12 in which saidresilient hinge means includes cut-out portions in the contact arm alonga transverse line between the contact support portion and intermediateportion thereby forming a line of weakness at which said resilient hingeaction is localized.

14. A contact arm subassembly according to claim 11 in which the contactsupport and base portions are separate members, and said resilient hingemeans includes an angularly disposed flange at the inner end of thecontact support portion and a similar flange at the outer end of theintermediate portion, said flanges extending parallel and adjacent toeach other, and means fastening said flanges together.

15. A contact arm subassembly according to claim 14 in which saidfastening means includes means electrically insulating said portionsfrom each other.

16. A contact arm subassembly according to claim 11 in which saidintermediate portion includes longitudinally extending deformed portionspreventing substantial flexing of said intermediate portion except alonga transverse hinge axis between the base portion and intermediateportion.

17. A thermal switch comprising first and second contact arms carryingfirst and second contacts respectively, the first contact arm having abase portion and an auxiliary portion, means movably mounting theauxiliary portion with respect to the base portion for relative movementbetween a first position in which the first contact is relatively nearerthe second contact and a second position in which the first contact isdisplaced from its first position in a direction away from the secondcontact, and

a longitudinally expanding and contracting restraining member connectedbetween said auxiliary portion and said base portion and controlling therelative movement of the auxiliary portion, said restraining memberhaving a portion of substantial length and small cross-section connected in circuit for passage of current through said lastmentionedportion when the contacts are engaged, said cross section being smallwith respect to the current passed therethrough thereby causingsubstantial heating and longitudinal expansion of the restraining memberin response to engagement of the contacts and substantial cooling andcontraction of the restraining member in response to sep aration of thecontacts, said expansion of the restraining member acting in a directionto effect movement of the auxiliary portion from second to firstposition and said contraction of the restraining member acting in adirection to effect movement of the auxiliary portion from first tosecond position.

18. A thermal switch comprising first and second con tact arms carryingfirst and second contacts respectively, the first contact arm having anextension portion sup ported for limited movement toward and away fromthe second contact arm between first and second positions, said portionbeing resiliently biased toward its first position, and a restrainingmember connected under longitudinal tension between the extensionportion and another part of the switch and normally holding said portionagainst said resilient bias, a portion of said restraining member beingelectrically connected in circuit with one of said contacts and having asubstantial length and relatively small cross section such that passageof current through said restraining member portion on engagement of thecontacts expands said restraining member portion substantially andthereby permits resiliently biased movement of said extension portiontoward said second position, increasing the contact pressure anddelaying the subsequent disengagement of the contacts.

19. A contact arm subassembly for a thermal switch comprising a flxibleswitch arm carrying one contact of the switch and capable of deflectionto move the contact back and forth in a direction generally normal tothe plane of the arm, a contact positioning portion movably mounted onthe switch arm for limited movement between first and second positionswhich are displaced from each other in said direction generally normalto the plane of the arm, means resiliently biasing said contactpositioning portion from its second towards its first position, and arestraining member connected in longitudinal tension between the contactpositioning portion and another portion of the switch arm, saidrestraining member (when cold) having a length normally holding thecontact positioning portion in its second position against the urging ofsaid resilient biasing means, means connecting a portion of saidrestraining member electrically in circuit with said one contact, andsaid restraining member portion having a substantial length and a smallcross section with respect to the current to be passed through it,passage of such current thereby causing substantially rapid heating andexpansion of the member for resiliently biased movement of the contactportion from second to first position.

No references cited.

